The present invention is directed to permanent magnet machines, and more particularly to a method of making a permanent magnet machine more fault-tolerant.
Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly.
Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than rely solely on the HP engine spool to drive the electrical generators. The low-pressure (LP) engine spool provides an alternate source of power transfer.
PM machines (or generators) are a possible means for extracting electric power from the LP spool. However, aviation applications require fault tolerance, and as discussed below, PM machines can experience faults under certain circumstances and existing techniques for fault tolerant PM generators suffer from drawbacks, such as increased size and weight.
Permanent magnet (PM) machines have high power and torque density. Using PM machines in applications wherein minimizing the weight is a critical factor is therefore advantageous. These applications are wide ranging and include aerospace applications.
One of the key concerns with using PM machines is fault-tolerance since the magnets cannot be “turned off” in case of a fault. Traditionally, the use of PM machines has been avoided in applications where fault-tolerance is a key factor. When PM machines have been used in such applications, fault-tolerance has been achieved by paying a penalty in the form of oversized machines and/or converter designs, or using a higher number of phases which complicates the control process and adds to the overall system weight and cost.
As is known to those skilled in the art, electrical generators may utilize permanent magnets (PM) as a primary mechanism to generate magnetic fields of high magnitudes. Such machines, also termed PM machines, are formed from other electrical and mechanical components, such as wiring or windings, shafts, bearings and so forth, enabling the conversion of electrical energy from mechanical energy, where in the case of electrical motors the converse is true. Unlike electromagnets which can be controlled, e.g., turned on and off, by electrical energy, PMs always remain on, that is, magnetic fields produced by the PM persists due to their inherent ferromagnetic properties. Consequently, should an electrical device having a PM experience a fault, it may not be possible to expediently stop the device because of the persistent magnetic field of the PM causing the device to keep operating. Such faults may be in the form of fault currents produced due to defects in the stator windings or mechanical faults arising from defective or worn-out mechanical components disposed within the device. Hence, the inability to control the PM during the above mentioned or other related faults may damage the PM machine and/or devices coupled thereto.
Further, fault-tolerant systems currently used in PM machines substantially increase the size and weight of these devices limiting the scope of applications in which such PM machines can be employed. Moreover, such fault tolerant systems require cumbersome designs of complicated control systems, substantially increasing the cost of the PM machine.
In view of the foregoing, it would be advantageous and beneficial to provide a method for limiting winding currents for all types of faults, especially a turn-to-turn fault associated with a PM machine to significantly improve the fault-tolerance capability of the PM machine without substantially increasing the size, weight and/or complexity of the PM machine.